1. Field of the Invention
The present invention relates to a vehicle body frame for a saddle-ride vehicle, and to a motorcycle including the vehicle body frame.
2. Description of the Background Art
In many cases, vehicle body frames of saddle-ride vehicles are manufactured by welding together sections of pipe materials and tubular frames.
Two frame members may be joined either by butt welding or by fillet welding (see Patent Document 1 (FIG. 10), for example).
FIG. 10(a) of Patent Document 1 shows a cross section of a vehicle body frame. A stiffener gusset (60) (numbers in parenthesis represent ref. no.s described in the cited document; the same applies below) is joined to a down pipe (40) by welding. Specifically, end portions (65a, 66a) of the stiffener gusset (60) are joined to the down pipe (40) with weld beads illustrated in black.
Each of the weld beads has a substantially triangular cross section, and is therefore a bead formed by “fillet welding.” In fillet welding, the distance from the vertex of the right angle side of a right triangle to its hypotenuse is called “throat thickness.” The product of this “throat thickness” and the length of the weld (or the circumferential length in the case of circumferential welding) is the “weld area.” This weld area determines the “joint strength (or weld strength).”
The throat thickness is determined in proportion to the thickness of the end portion (65a, 66a). The circumferential length is determined by the size of the stiffener gusset (60).
When the required joint strength is large, such a requirement is fulfilled by increasing the thickness (and/or the size) of the stiffener gusset (60). In this way, however, the weight of the stiffener gusset (60) increases. As a result, the vehicle body frame becomes heavy, and therefore the vehicle becomes heavy.
There has been a strong demand for reducing vehicles' weights, driven in part by an effort to reduce fuel consumption. Therefore, welded vehicle body frames are also targeted for weight reduction.    [Patent Document 1] Japanese Patent Application Publication No. 2004-256073